In an adaptive, time varying system a predetermined impulse function used for excitation changes at every data sample point or set of data samples. Such a system is therefore nonlinear. A digital filter is directed to a particular transfer function by an adaption processor depending upon a desired function of the filter. An application of a digital filter is to achieve the minimum mean square error when filtering a set of data with a predetermined transfer function. This function is known as the least-mean-squares (LMS) algorithm. The error represents an error between some desired goal and what the filter is actually computing and is a measurement of the misadjustment of the filter. The LMS algorithm involves: (1) an adaption computation which is an update of coefficients in the filter based in part upon the previous error; (2) a convolution computation which is an accumulation of the product of predetermined coefficient values and data values; and (3) an error calculation.
One application of the LMS algorithm is within the implementation of a telecommunications U-interface transceiver as defined by the American National Standards Institute (ANSI) for a U-interface reference model. This interface is the twisted pair of wires which link a central office location and a subscriber in a communications system. The ANSI standard specifies a coding scheme, a data transmission rate, an echo cancellation technique and startup and synchronization procedures at both a network termination and a line termination. The LMS algorithm may be used to implement a variety of functions including: (1) a linear echo canceller (LEC); (2) a decision feedback equalizer (DFE); and (3) adaptive reference compensation. In addition, a general purpose multiplier is needed to implement a U-interface transceiver.
Others have developed a data processor for ISDN U-interface transceivers which conform to the ANSI standard. As an example, Koh et al. in an article entitled "Algorithms and Architecture of a VLSI Signal Processor For ANSI Standard ISDN Transceiver" in the IEEE International Conference of Acoustics, Speech and Signal Processing Proceedings, 1989, pages 2468-2471, a processor which implements a U-interface transceiver is shown. A plurality of individual arithmetic unit circuits is typically required to implement each of the functions of the linear echo canceller and decision feedback equalizer and separate circuitry is required to implement a general purpose multiplier. Similar subject matter is also discussed by Khorramabadi et al. in a paper entitled "An ANSI Standard ISDN Transceiver Chip Set" in the IEEE International Solid-State Circuits Conference, 1989, pages 256-257. Another example of a processor which implements the ISDN interface defined by ANSI is described by Agazzi et al. in a paper entitled "A Digital Signal Processor for an ANSI Standard ISDN Transceiver" in the IEEE Journal of Solid-State Circuits, Vol. 24, No. 6, Deccember 1989. A large amount of individual arithmetic processing units are typically required to implement each of the distinct functions required by the ANSI standard at the U-interface.